Conventional processes to form field oxides are shown in FIGS. 1 and 2. FIG. 1 shows a substrate 1 and two active regions 12 thereon. As shown in FIG. 2, pad oxides 14 are formed on the active regions 12, then nitrides 16 are formed on the pad oxides 14 as shields. After that, the substrate 1 is thermally oxidized to grow field oxides 10.
In conventional methods of forming the field oxides, the boundaries of the active regions 12 and nitrides 16 are straight. After oxidization, bird beaks 18 are formed between the active regions 12 and the field oxides 10. As a result, there are large stress induced by the bird beaks 18 in the active regions. Such stress results in crystalline defects which cause problems like junction leakage.